In recent years, downsizing and sophistication of electronic devices, such as portable electronic devices, demand printed wiring boards having further miniaturized (fine-pitched) wiring patterns. To meet such a requirement, a copper foil for manufacturing a printed wiring board is desired to have a smaller thickness and a lower surface roughness than current ones.
The copper foil for manufacturing a printed wiring board is used in lamination to an insulating resin substrate. It is important to ensure bonding strength between the copper foil and the insulating resin substrate because low bonding strength may cause traces to separate during manufacturing the printed wiring board, resulting in a low product yield. In this regard, usual copper foil for manufacturing printed wiring boards is subjected to a roughening treatment to form asperities on the surface to be bonded. The asperities are pressed to bite into the insulating resin substrate to exhibit an anchoring effect, resulting in improved adhesion. This technique involving the roughening treatment is, however, incompatible with the above-described copper foil aimed to meet the fine pitch and having a smaller thickness and a lower surface roughness than conventional ones.
A copper foil for a printed wiring board to improve the adhesion between the copper foil and the insulating resin substrate without roughening treatment is known. For example, Patent Document 1 (JP2007-266416A) discloses a metal foil having a surface roughness Rz of 2.5 μm or less on at least one side thereof, wherein at least Si oxide (such as SiO2) sites are scattered about and exposed on the one side. The metal foil actually used in this document is a rolled copper foil having a thickness of 18 μm and a surface roughness Rz of 0.7 μm on both sides.